Two problems involved in the pathogenicity of African trypanosomes are to be investigated: antigenic variation and the differentiation of bloodstream- to insect-forms. These phenomena will be approached at the molecular and biochemical levels through the detailed study of the relevent genes and their products. Four discrete sets of experiments are proposed, each with its own specific goal. 1. The biosynthesis of the molecules involved in antigenic variation, the variant-specific surface glycoproteins (VSGs), will be examined using specific antisera as reagents in a series of pulse/chase experiments and in situ localization by electron microscopy. 2. The evolution and activation of VSG genes will be investigated through studies on the structure and function of a highly conserved family of such genes. 3. The organization of the VSG gene transcriptional unit will be determined by molecular cloning of large genomic fragments in "cosmids" in an attempt to elucidate the genetic control of VSG gene expression and switching. 4. Differentiation of the bloodstream-forms to the insect-forms will be artificially established in vitro and used to examine the changes in gene expression associated with this transformation by comparing the structure of putative regulatory sequences in cloned stage-specific genes. All of these experiments deal with properties possessed by the parasite but absent from the host and so their elucidation may reveal pathways amenable to chemotherapeutic attack.